This invention relates to the field of distributorless electronic ignition systems and, more particularly, to systems wherein the desired advance angle is a function of other engine characteristics in addition to speed.
In an internal combustion engine the timing of the ignition spark is of extreme importance to the operation of the engine. Various mechanical arrangements have been designed, generally utilizing centrifugal force to provide spark advance with speed. Because such arrangements required readjustment from time to time and did not lend themselves to precise or sophisticated advance curves, electronic advance (distributorless) ignition systems have been developed.
In one of these, described in U.S. Pat. No. 4,104,997, a "programmable" voltage slope generator, including a capacitor and various current sources, produces a signal voltage having more than one rate of change. An engine rotation sensor and timer combination alter the rate of change of voltage in response to varying engine speeds. A vacuum sensor also provides an input to the programmable generator in response to changes in engine load. The voltage on a capacitor in the generator is compared with a reference voltage and when the levels are equal, a spark timing pulse is produced which enables the firing of the appropriate cylinders. While this analog approach is a big improvement over mechanical arrangements, even greater accuracy has proven desirable.
In a subsequent U.S. Pat. No. 4,168,682, a digital circuit was shown wherein signals from various engine condition sensors produce a train of pulses representing the desired angle of advance for each instantaneous combination of engine conditions. These pulses are multiplexed and coupled to a binary rate multiplier (BRM) which outputs to a counter. A second counter monitors the input to the BRM and controls the output of a ROM which controls the multiplication rate of the BRM. The BRM output then controls the input to a phase counter which, in turn, controls the actual spark timing.
Still another U.S. Pat. No. 4,231,332, describes an improvement upon the circuits of the two previous patents. (All three of the patents referred to herein are assigned to the assignee of the present invention.) In this latter circuit, again sensor pulses are combined and inputted to a ROM which also receives the crankshaft position pulses and controls a BRM which, in turn, inputs to a series of counters. The accumulated count then determines the timing of the ignition spark. This system utilizes less storage in the ROM than the previous version, '682, but still more than is desirable. While all of the above-described systems provide an increase in accuracy of the advance angle calculation, all leave room for improvement.